> Wouldn't that need a huge amount of extra hardware to do that filtering 20 years ago Cisco (probably much longer) routers were able to do this without notice

> Wouldn't that need a huge amount of extra hardware to do that filtering 20 years ago Cisco (probably much longer) routers were able to do this without noticeable performance overhead (ip verify unicast reverse-path). I don't think modern routers are worse. Generally filtering is expensive if you need a lot of rules which is not needed here.